The present invention relates to a method of testing electronic components and, more particularly, to a method of judging carrier lifetime in semiconductor devices.
Semiconductor materials such as silicon and germanium have an atomic structure in which the outer or valence shell has four electrons. A completely filled valence shell has a total of eight electrons. Since the valence shell has exactly half the number of electrons of a filled valence shell, an atom of such semiconductor can enter into a crystalline latticework structure with four neighboring atoms. Due to covalent bonding, each of the four other atoms shares one of their valence shell electrons with the first atom to effectively fill the valence shell of the first atom. Likewise, the first atom shares one of its valence shell electrons with each of the four neighboring atoms. In a pure crystalline structure, each of the four other atoms similarly shares electrons so that theoretically the valence shell of each atom is filled due to covalent bonding.
Unlike conductors, which have many free electrons, and insulators, which have virtually no free electrons, pure semiconductor materials have a few free electrons, and the number of such electrons increases with temperature. This occurs because of the moderate gap energy needed for an electron in a semiconductor to jump from the valence shell to the conduction level where it is available to transport electron current. A valence electron in an insulator such as diamond requires a gap energy of 7 electron volts (eV) while a valence electron in a conductor such as tin needs only 0.1 eV. By contrast, the gap energy for silicon is 1.1 eV and for germanium is 0.72 eV.
At any temperature above absolute zero, there are a few naturally occurring thermally generated free (conduction level) electrons present in a semiconductor. When an electron, having accumulated sufficient energy, jumps from the valence shell, it leaves behind a hole and thus causes the atom which lost the electron to become a positively charged ion. It may happen that an electron from the conduction level moves to fill the hole in the atom thus causing the negative charge of the electron and the positive charge of the ion to cancel one another. This process is known as recombination.
An n type semiconductor has extra free electrons and is made by doping a pure intrinsic semiconductor crystal with donor impurities having atomic structures with five electrons in their valence shell. Commonly used pentavalent donors are phosphorous, arsenic, antimony and bismuth, and a semiconductor formed by doping is known as an extrinsic semiconductor. When a donor atom enters into covalent bonding with the four silicon or germanium atoms, a free electron is formed because there are nine electrons available for a valence shell which can hold a maximum of eight, and the donor impurity, having lost an electron, becomes a positively charged ion.
A p type semiconductor has an excess of holes and is formed by doping the pure semiconductor crystal with acceptor impurities having atomic structures with three electrons in their valence shell. Commonly used trivalent acceptors are boron, aluminum, gallium and indium. When an acceptor atom enters into covalent bonding with the four other atoms, a hole is formed because even with covalent bonding the acceptor atom has only seven electrons in its valence shell. However, if a free electron moves to fill the hole thus formed, the acceptor atom becomes a negatively charged ion.
When standing alone and in the absence of any electrical field, a p type semiconductor has an even distribution of holes and negatively charged acceptor ions. Similarly, under the same circumstances an n type semiconductor has an even distribution of free electrons and positively charged donor ions. However, when a p type region and an n type region are formed next to each other in the same semiconductor crystal, these even distributions are radically altered. In the vicinity of the boundary or junction formed between the p region and the n region, the holes and electrons diffuse toward one another and combine. This results in the formation of a depletion region including the pn junction. The term "depletion" is quite appropriate as this region is depleted of current carriers (holes or electrons).
The p side of the depletion region contains negative ions while the n side of the region contains positive ions. The ions in the depletion region cannot carry current because they cannot move since they are fixed in the covalent bonding of the crystal lattice. Since there are no carriers in the region capable of moving, the depletion region becomes a barrier to current flow. Holes in the p region are repulsed by the positive ions on the n side of the depletion region, and electrons in the n region are repulsed by the negative ions on the p side of the depletion region.
The width of the depletion region can be changed by the application of an external voltage to the pn junction. The junction can be forward-biased to reduce the width of the depletion region so that holes and electrons can pass through it, thus rendering the semiconductor conductive. On the other hand, reverse-biasing of the junction increases the width of the depletion region to preclude passage of substantial numbers of electrons and holes, thus causing the semiconductor device to function as an insulator.
Conduction in semiconductor material occurs not only because of the diffusion or charge motion from a region of higher density to a region of lower charge density. Additionally, conduction occurs because of the drift of holes or electrons under the influence of an applied potential. As mentioned above, there are naturally occurring thermally generated electron-hole pairs in intrinsic semiconductors and the same is true of extrinsic or doped semiconductors. In an n type semiconductor electrons are called the majority carriers since they predominate. However, in an n type semiconductor thermally generated carriers of the opposite type (holes) are also present in small quantities and are known as minority carriers. Of course, in a p type semiconductor, holes are the majority carrier and electrons are the minority carrier. The resistivity of the semiconductor is a function of the concentration of majority carriers.
Additional minority carriers can be injected into the semiconductor material by various means including applying an electrical pulse to the material or by illuminating the material with a short light pulse. These additional minority carriers, over and above the number of naturally occurring, thermally generated minority carriers, are known as excess minority carriers. With the passage of time, these excess minority carriers recombine with the majority carriers. Carrier lifetime is the average time required for excess minority carriers to recombine with the majorities and usual lifetimes are in the range of 1 to 1,000 microseconds. In some applications of semiconductor devices short carrier lifetime is desired because the carrier lifetime determines the turn-off time of the device. That is, excess or stored minority carrier densities must be depleted before a diode will cease to conduct after having been reversely biased. Judgment of carrier lifetime is a way of checking the performance of semiconductor devices at the production line of such devices.
One method of measuring minority carrier lifetime in semiconductor materials employs a noncontacting steady-state technique wherein a light source is switched on and off at a frequency which is low compared to the reciprocal of the longest acceptable carrier lifetime. The conductivity of the semiconductor is measured by contactless means both during periods of illumination and during periods of nonillumination. The difference in conductivity during the respective periods is proportional to the effective carrier lifetime. For a further description of this method and the apparatus used for implementing it, reference may be made to U.S. Pat. No. 4,286,215.
In another known method, the doping density profile or impurity distribution of a semiconductor wafer is measured. In this method, an oscillating voltage of frequency f is applied to a reverse-biased semiconductor diode, and the output alternating voltage derived from the diode is examined. This method is predicated on the discovery that if alternating current of frequency f is directed through a reverse-biased semiconductor diode, the amplitude of the output voltage at the fundamental frequency is proportional to the depth of the semiconductor depletion layer and that the amplitude of the voltage at the second harmonic frequency 2f will be proportional to the reciprocal of the doping density at the edge of the depletion layer. For a further description of this method and apparatus used in carrying it out, reference may be made to U.S. Pat. No. 3,518,545.
In still another known method, excess carrier lifetime is judged by measuring the slope of an open-circuit voltage decay curve taken across the device. In this method, rectangular current pulses are repetitively applied to the device to be tested. The slope of the decay curve is measured by observing the voltage across the device immediately following the termination of each current pulse. It will be appreciated that in this method, as in the previouslydiscussed methods, quantification is required. Accordingly, rather complex circuitry and equipment is required to carry out the method. See U.S. Pat. No. 4,090,132 for a further description of this method and the equipment required to implement it.